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Shell, Terminal, Bash & Kernel in Linux: Definition & Basic Commands

Instructor: Alexis Kypridemos

Alexis is a technical writer for an IT company and has worked in publishing as a writer, editor and web designer. He has a BA in Communication.

This lesson explains the shell, terminal, bash, and kernel in Linux, and how these work in layers for the user to enter commands. Also discussed are some basic shell commands for you to practice.

The Linux Kernel

Every operating system (OS) has a kernel. The kernel is the layer of the OS that bridges the hardware with the main programs that run on a computer. The kernel is the core of the OS and is the first to load when the computer boots up. It remains in the computer's memory throughout a session. It is responsible for providing an interface for all applications, controlling the hardware and allowing processes to get information from each other. There are three types of kernel:

  • Microkernel
  • Hybrid
  • Monolithic

Linux uses a monolithic kernel, so called because it includes device drivers and the file management system and therefore requires more memory. On the opposite end of the spectrum, a microkernel aims to occupy the smallest amount of memory by managing only the necessities such as the CPU, memory and inter-process communication (IPC).

The Shell

The shell forms the layer between the user and the kernel so the user can enter commands. The kernel 'understands' only binary language, which is composed exclusively of ones and zeros. In early computing, any instructions/commands from the users were entered in binary language, but this evolved so that the user can enter commands in a more recognizable language. It is the shell that acts as the go-between, accepting the commands entered in the language recognizable by the user, and translating them to binary language for the kernel.

Bash

Bash stands for Bourne Again SHell, and is a type of shell found in Linux, which is the default shell in several versions ('distributions') of Linux. Other common types of shell are cshell and kshell, though there are others. The most primitive type of shell in Linux is sh.

Terminal

The terminal is the application that brings it all together, in the sense that it provides a visual representation of the shell for the user to enter commands. In other words, in a GUI (graphical user interface), where applications and other features are visually represented by images that the user can manipulate by clicking on them with a cursor, a terminal application opens a window where the user can type in commands for the shell to interpret into binary language for the kernel.

Summing it all up, we could say that we've discussed three layers so far: the terminal, where the user enters written commands; the shell, and Bash being a type of shell, which takes those commands and interprets them into binary language; the kernel which takes the binary language commands and executes the task on hand.

Some Basic Commands

The following are some basic commands that can be entered in a terminal in Linux:

ls - Lists the contents of a directory (folder). For example, the command 'ls Folder' might produce a list like 'document1.txt image1.jpg'.

cat - Lists the contents of a file. For example, the command 'cat document.txt' would actually show in the terminal the contents of the 'document.txt' text file.

mv - Moves a file. For example, the command 'mv Documents/document.txt Downloads' will move the file 'document.txt' from the Documents folder to the Downloads folder.

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